Magnetic core memory consists of tiny donuts of ferrite material strung on wires into arrays. Each donut could store a bit, and the value of the bit (zero or one) was identified by the direction of their magnetic flux. The wires that ran through the holes in the donuts could both detect (that is, read) and change (that is, write) the magnetization of the cores. Core memory became the dominant memory technology during the first two decades of the Cold War. But manufacturing it was a delicate job. The cores were tiny and had to be threaded by steady hands using magnifying glasses. As cores got smaller, engineers joked that new cores were made from the holes punched out of the previous generation of cores.